Pretty much everyone is familiar with the most famous member of the Bluegrass family, Kentucky Bluegrass (Poa pratensis). Considered the Cadillac of lawn grasses, Kentucky Bluegrass is the most widely used turf species in the U.S. thanks to its dark-green color, lush texture, and thick spreading growth habit. And like some families, the Poa family has a black sheep. Poa annua, commonly known as Annual Bluegrass, is a bit of a misfit in the Bluegrass family. Unlike its more well-known cousin, Annual Bluegrass is not desirable in lawns. In fact, it’s considered a stubborn weed that can spoil the consistency, color, and uniformity of an otherwise beautiful lawn. While Annual Bluegrass is a difficult weed to control and almost impossible to completely eradicate, it is possible to manage the population down to a tolerable level by following some simple cultural control practices.
What Does it Look Like, and Why is it Such a Problem?
Annual Bluegrass is a clump-forming cool-season annual grass that begins germinating in autumn, goes dormant during winter, and ends its life cycle the following spring. The problems associated with Annual Bluegrass become very apparent as it begins to grow and spread in Kentucky Bluegrass lawns. Color differences are usually the first thing that’s noticed. Unlike the deep, dark-green color of Kentucky Bluegrass, Annual Bluegrass has a much lighter, “green-apple” appearance that stands out. When left unmowed, Annual Bluegrass will grow 6 to 8 inches high, but thrives at shorter lengths. This makes it especially annoying on golf courses where it invades putting greens and disrupts the smooth surface. On top of these problems, Annual Bluegrass has a coarser leaf texture and produces unsightly seedheads. Each plant can develop and drop up to 100 seeds in just a couple months after germinating, giving this prolific seeder its true weed status.
Avoid Compacted Soils
It’s important to understand the conditions that will contribute to Annual Bluegrass growth in order to avoid infestations. First, Annual Bluegrass loves compacted soil. Regular core aerating helps eliminate this condition and improves the overall health and vigor of the lawn. It’s best to core aerate before Annual Bluegrass germinates in the autumn so cool-season lawns can recover as quickly as possible before Annual Bluegrass germination begins.
Second, Annual Bluegrass thrives in wet soil conditions. I came to understand this all too well during the summers I managed the estate of a high-profile client. At the end of their driveway they had a beautiful water feature that ensured the surrounding soil stayed constantly moist. It was in this area I came to know firsthand the annoyance of Poa annua. This was a unique situation, but in most instances this can be solved by watering deeply and infrequently instead of lightly and frequently. Overwatering is not only wasteful, but it provides Annual Bluegrass with perfect habitat.
Avoid Applying Excessive Fertilizer
Next, avoid applying excessive nitrogen fertilizer during peak Annual Bluegrass germination periods. Annual Bluegrass responds well to high nitrogen applications. These applications only help it spread and better its chances of surviving into winter and spring. Better yet, use a slow-release organic maintenance fertilizer instead of synthetic fertilizers. These organic fertilizers apply nutrients slowly and over time instead of short bursts. They also help improve the amount of organic matter in the soil.
Mow Lawn Higher During Peak Annual Bluegrass Season
Finally, mow your grass a little higher during peak germination times. Leaving grass just a bit longer will help shade and crowd out Annual Bluegrass. On the other hand, mowing your lawn short will stress your grass and encourage Annual Bluegrass growth. Also, while I usually recommend leaving grass clippings on the lawn to act as a natural fertilizer, when Annual Bluegrass is present it’s best to bag grass clippings. Removing grass clippings during Annual Bluegrass outbreaks will help stop the spread of weed seed.
Chemical Control as a Last Resort
Chemical control should be a last option and only for extreme problem areas. Preemergent herbicides offer some prevention, but will not get rid of existing plants. For preemergent herbicides, timing is crucial and depends on climate, temperature, and other factors. Look for a preemergent herbicide containing the active ingredients benefin and trifluralin. This should be applied in autumn generally when the daily high temperature drops to 75 degrees for several days. A second application can be applied 10 to 12 weeks later. For individual plants, a non-selective herbicide containing glyphosate will kill Poa annua, but also any other plant it touches. This option should only be used in pure stands of Annual Bluegrass or when starting over is the best decision. Remember, non-chemical solutions should always be your first choice.
We all have a few interesting characters somewhere in our family tree, and the Bluegrass family is no different. By following these simple practices we can ensure our lawns are not overrun by the weedy pest, Annual Bluegrass.